Reversible colour changes of mixed films composed of 4,4′-bis[bis(4-methylphenyl)amino]azobenzene and organic acids in response to exhaled breath

We found that mixed films made of the title aminoazobenzene-based material with various organic acids exhibited reversible colour changes in response to exhaled breath. Such reversible colour changes were shown to be general phenomena for films composed of aminoazobenzene-based amorphous molecular materials mixed together with organic acids. The ability of this type of film to reversibly change colour was suggested to be due to protonation of the aminoazobenzene caused by the organic acid and deprotonation by moisture, and the acidity levels and concentrations of the organic acids were suggested to be important factors affecting the colour of the film and its response to moisture. Mixed films of the title aminoazobenzene with a few different conventional organic acids exhibited drastic and reversible colour changes when exposed to exhaled breath.